The present invention generally relates to enhanced efficiency antioxidants and antioxidant boosters. The unique antioxidants are preferably formed such that they contain a predetermined amount of unconverted hydroxyl groups which produce the hydroperoxyl radical or moiety (HOO.) upon oxidation. The hydroperoxyl radical reacts with alkoxyl radicals (RO.) or alkylperoxyl radicals (ROO.) to produce stable intermediates and molecular oxygen. The antioxidants of the present invention can be used by themselves, grafted onto conventional antioxidants to act as antioxidant boosters, or grafted onto other materials such as mineral oils, polyol esters, polyalpha olefin base stocks, fuels, oligomers and polymers so as to provide antioxidant protection.
Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone oils, diesters and polyol esters.
Polyol esters have been commonly used as base stocks in lubricant application where thermal and oxidative stability are critical. Despite their inherent thermal/oxidative stability as compared with other base stocks (e.g., mineral oils, polyalpha olefins, etc.), even these synthetic ester lubricants are subject to oxidative degradation and cannot be used, without further modification, for long periods of time under oxidizing conditions. It is known that this degradation is related to oxidation and hydrolysis of the ester base stock.
Conventional synthetic polyol ester lubricant oil formulations require the addition of antioxidants (also known as oxidation inhibitors). Antioxidants reduce the tendency of the ester base stock to deteriorate in service in which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on the metal surfaces, and by viscosity and acidity growth. Such antioxidants include arylamines (e.g., dioctyldiphenylamine (V-81) and phenylalphanaphthylamine), and the like.
Frequently replacing the lubricant oil or adding an antioxidant thereto to suppress oxidation increases the total cost of maintaining the engine or machine. It would be most desirable to either enhance the efficiency of conventional antioxidants (i.e., provide increased antioxidant potency in formulated products) or produce an ester base stock which exhibits substantially enhanced thermal/oxidative stability compared to conventional synthetic ester base stocks, and wherein the ester base stock does not require frequent replacement due to decomposition (i.e., oxidation degradation). It would also be economically desirable to eliminate or reduce the amount of antioxidant which is normally added to such lubricant base stocks.
Upon thermal oxidative stress a weak carbon hydrogen bond cleaved ultimately resulting in an unstable alkylperoxyl radical on the ester. The role of conventional antioxidants is to transfer a hydrogen atom to the unstable carbon radical and effect a xe2x80x9chealingxe2x80x9d of the radical. The following equation demonstrates the effect of antioxidants (AH):
AH+ROO.xe2x86x92A.+ROOH 
The antioxidant molecule is converted into a radical, but this radical (A.) is far more stable than that of the ester-based system. Thus, the effective lifetime of the ester is extended. When the added antioxidant is consumed, the ester radicals are not healed and oxidative degradation of the polyol ester composition occurs. One measure of relative thermal/oxidative stability well known in the art is the use of high pressure differential scanning calorimetry (HPDSC).
HPDSC has been used to evaluated the thermal/oxidative stabilities of formulated automotive lubricating oils (see J. A. Walker, W. Tsang, SAE 801383), for synthetic lubricating oils (see M. Wakakura, T. Sato, Journal of Japanese Petroleum Institute, 24 (6), pp. 383-392 (1981)) and for polyol ester derived lubricating oils (see A. Zeeman, Thermochim, Acta, 80(1984)1). In these evaluations, the time for the bulk oil to oxidize was measured; this is the induction time. Longer induction times have been shown to correspond to oils having higher concentrations of antioxidants or correspond to oils having more effective antioxidants or at a fixed level of a given antioxidant, have been shown to correspond to oils having intrinsically more stable base stocks. For automotive lubricants, higher induction times have been correlated with viscosity break point times.
The use of HPDSC as described herein provides a measure of stability through oxidative induction times. A polyol ester can be blended with a constant amount of dioctyl diphenylamine which is an antioxidant. This fixed amount of antioxidant provides a constant level of protection for the polyol ester base stock against bulk oxidation. Thus, oils tested in this manner with longer induction times have greater resistance to oxidation. For the high hydroxyl esters of the present invention in which no antioxidant has been added, the longer induction times reflect the greater stability of the polyol ester high hydroxyl molecules by and also the natural intrinsic antioxidancy of the esters due to the unesterified xe2x80x94CH2OH groups.
The present inventors have developed a unique polyol ester composition having enhanced thermal/oxidative stability when compared to conventional synthetic polyol ester compositions. This was accomplished by synthesizing a polyol ester composition from a polyol and branched and/or linear carboxylic or aromatic acid in such a way that it has a substantial amount of unconverted xe2x80x94CH2OH groups. Having a highly branched polyol ester backbone enhances the ability of the high hydroxyl ester to act similarly to an antioxidant, i.e., cause the thermal/oxidative stability of the novel polyol ester composition to drastically increase, as measured by high pressure differential scanning calorimetry (HPDSC). This selectively branched polyol ester composition restricts the mechanisms by which the non-CH2OH portion of the high hydroxyl ester oxidizes and thereby provides an intramolecular mechanism which is capable of scavenging alkoxide and alkyl peroxide radicals, thereby substantially reducing the rate at which oxidative degradation can occur.
The thermal and oxidative stability which is designed into the novel polyol ester compositions of the present invention eliminates or reduces the level of antioxidant which must be added to a particular lubricant, thereby providing a substantial cost savings to lubricant manufacturers. Furthermore, the novel high hydroxyl polyol esters of the present invention are capable of decreasing the level of other antioxidants required when they are grafted to other formulated products including lubricants, fuels, oligomers and polymers, thus potentially reducing the cost of formulation to achieve desired performance targets.
Another problem of conventional formulations is that the ability to dissolve conventional antioxidants in the respective base stock varies widely and in some instances the use of the selected antioxidants is prohibited due to solubility limitations. The present invention also offers a means of incorporating additives into formulations that may be poorly soluble by attaching them through conventional hydrocarbon bonds to the additive in question. The high hydroxyl ester thus provides not only solubility but provides antioxidancy to the additive in question.
The high hydroxyl polyol esters of the present invention, when grafted onto oligomers, polymers or the like are capable of providing the enhancement of oxidation stability without the potential debits associated with conventional antioxidants that are known to produce color bodies, a significant debit for selected polymer applications.
The present invention also provides many additional advantages which shall become apparent as described below.
The antioxidant of the present invention is preferably a unique polyol ester having unconverted hydroxyl groups from the reaction product of a polyol with a branched and/or linear carboxylic or aromatic acid, thereby allowing the unconverted xe2x80x94CH2OH groups to form the hydroperoxyl moiety (HOO.) upon oxidation. This antioxidant comprises a primary aliphatic hydroxyl group linked to an organic backbone such that upon oxidation a hydroperoxyl moiety capable of boosting the efficacy of free radical scavengers and hydroperoxide decomposer type antioxidants is generated in situ.
Antioxidants that are covered by the present invention include the general class of polyol esters, phenols, aromatic amines, phosphites, and sulfur containing antioxidants, as well as any conventional antioxidants that function as free radical inhibitors or hydroperoxide decomposers, provided that these antioxidants contain an unconverted xe2x80x94CH2OH group which is capable of forming a hydroperoxyl radical or moiety (HOOxe2x80xa2). Depending upon the structure of the acid used, the resultant ester product can function as either an antioxidant or an antioxidant booster.
The antioxidant comprises the reaction product of: a branched or linear alcohol having the general formula R(OH)n, wherein R is an aliphatic or cyclo-aliphatic group having from about 2 to 20 carbon atoms and n is at least 2; and at least one branched or linear carboxylic acid which has a carbon number in the range between about C2 to C20; wherein said antioxidant has a hydroxyl number greater than about 30 to about 180 and preferably between about 40 and 120. The unconverted hydroxyl group of this antioxidant is preferably reacted with a facile leaving group, thereby producing free xe2x80x94CH2OH groups upon hydrolysis or oxidation of the chemically protected derivative.
Alternatively, the antioxidant of the present invention includes the modification of general antioxidants (AH) through grafting reactions that result in new antioxidants which produce free xe2x80x94CH2OH groups upon hydrolysis and/or oxidation. Such antioxidants formed through grafting of general antioxidants (AH) to form a reaction product comprising an unconverted xe2x80x94CH2OH moiety. The preferred reaction product comprises the reaction of YH with a polyol diacrylate, wherein Y is selected from the group consisting of: RS, R2N, (RO)2PHO, phenols and mixtures thereof; provided that RS is part of a sulfur containing antioxidant, R2N is an aromatic amine or an alkyl amine antioxidant, and (RO)2PHO is a phosphite antioxidant.
Alternatively, the antioxidant of the present invention also includes antioxidants that do not contain any free hydroxyl groups nor any chemical bonds that would produce the xe2x80x94CH2OH moiety upon hydrolysis, but which do produce the hydroxyl group upon oxidation. Such an antioxidant would also have boosted antioxidant capacity due to the subsequent oxidation reaction of the xe2x80x94CH2OH moieties to yield the hydroperoxyl moiety (HOO.). The hydroperoxyl radical or moiety can then react with alkylperoxyl radicals or with alkylhydroperoxyl radicals to produce stable intermediates (e.g., ROOH) and molecular oxygen. Such antioxidant can be derived from the reaction product of: (1) tris (hydroxymethyl)aminomethane and formaldehyde, thereby producing 1-aza-3,7-dioxa-bicyclo[3.3.0]octyl-5-methanol; (2) tris (hydroxymethyl)aminomethane and cyclohexanone, thereby producing 2,2-pentamethylene-1,3-oxazolidine-4,4-dimethanol; (3) tris (hydroxymethyl)aminomethane and a linear or branched carboxylic acid, thereby producing amides having the general formula:
R(Cxe2x95x90O)NHC(CH2OH)3 
wherein R is a linear or branched alkyl chain derived from the reaction product of a linear or branched acid with tris (hydroxymethyl)aminomethane; and (4) tris (hydroxymethyl)aminomethane and a linear or branched carboxylic acid, thereby producing substituted oxazolines having the general formula: 
wherein R is a linear or branched alkyl chain derived from said linear or branched carboxylic acid.
The unique high hydroxyl polyol esters antioxidants of the present invention, when grafted onto mineral oils, polyol esters, polyalpha olefin base stocks, fuels, oligomers and polymers can increase the oxidation stability of those materials as well.
The present invention also includes a lubricant which is prepared from at least one synthetic polyol ester composition having unconverted hydroxyl groups as set forth immediately above and a lubricant additive package. Additionally, a solvent may also be added to the lubricant, wherein the lubricant comprises about 60-99% by weight of the synthetic polyol ester composition, about 1 to 20% by weight the additive package, and about 0 to 20% by weight of the diluent.
The lubricant is preferably one selected from the group consisting of: crankcase engine oils, two-cycle engine oils, catapult oils, hydraulic fluids, drilling fluids, turbine oils, greases, compressor oils, gear oils, and functional fluids.
The additive package comprises at least one additive selected from the group consisting of: viscosity index improvers, corrosion inhibitors, oxidation inhibitors, dispersants, lube oil flow improvers, detergents and rust inhibitors, pour point depressants, anti-foaming agents, anti-wear agents, seal swellants, friction modifiers, extreme pressure agents, color stabilizers, demulsifiers, wetting agents, water loss improving agents, bactericides, drill bit lubricants, thickeners or gellants, anti-emulsifying agents, metal deactivators, and additive solubilizers.
Still other lubricants can be formed according to the present invention by blending this unique synthetic high hydroxyl polyol ester composition of the present invention and at least one additional base stock selected from the group consisting of: mineral oils, highly refined mineral oils, poly alpha olefins, polyalkylene glycols, phosphate esters, silicone oils, diesters and polyol esters. The synthetic polyol ester composition is blended with the additional base stocks in an amount between about 1 to 50 wt. %, based on the total blended base stock, preferably 1 to 25 wt. %, and most preferably 1 to 15 wt. %.